It is a positive Torah commandment to eliminate all ḥametz from our possession before Pesaḥ, as it is written, “Yet on the first day you must remove the se’or from your homes” (Shemot 12:15). The oral tradition teaches that we are to clear out the ḥametz by midday of the 14th of Nisan, Erev Pesaḥ. This ruling is supported by the verse, “You shall not slaughter the blood of My sacrifice over ḥametz” (ibid. 34:25), which is interpreted to mean that one may not slaughter the Paschal sacrifice while there is still ḥametz in his possession, and the time for slaughtering the Paschal sacrifice begins at midday on the 14th of Nisan (see Pesaḥim 4b; MT, Laws of Ḥametz and Matza 2:1). Men and women are equally obligated in this mitzva, as they are concerning all of the mitzvot of Pesaḥ.
Whoever did not remove the ḥametz from his home by midday of the 14th of Nisan is in violation – every single moment that he delays – of the positive commandment of removing the ḥametz (MB 443:1). Furthermore, from the moment the Pesaḥ holiday begins, he is in violation of two prohibitions: bal yimatzei, as it is written, “Seven days there shall be no se’or found in your homes” (Shemot 12:19), and bal yera’eh, as the Torah declares, “Matzot shall be eaten seven days; and no ḥametz of yours shall be seen, and no se’or of yours shall be seen within all your borders” (ibid. 13:7). Thus, by fulfilling the mitzva of eliminating ḥametz, we are saved from two prohibitions: bal yera’eh and bal yimatzei – that ḥametz should be neither seen nor found in our possession.
The mitzva of removing the ḥametz is the first in a series of mitzvot connected with Pesaḥ. As noted, ḥametz on Pesaḥ is a metaphor for the evil inclination, and one has to clear out the ḥametz from the house in order to experience the sanctity of the Pesaḥ sacrifice and the eating of the matza properly. Therefore, the first of the preparations for Pesaḥ is removal of the ḥametz.
One who participated in the Paschal sacrifice (korban Pesaḥ) while he still had at least a kezayit of ḥametz in his possession violated a Torah prohibition, as it says: “You shall not slaughter the blood of My sacrifice over ḥametz” (Shemot 34:25). If he was warned about this and still did it on purpose, he incurs the penalty of lashes (MT, Laws of Korban Pesaḥ 1:5).